TDR (Time-Domain-Reflectometry) measuring devices are used increasingly for detecting fill level of liquids or bulk goods in containers. In the TDR measurement method, electromagnetic, high-frequency pulses or continuous microwaves are guided into, respectively out of, the container along a conductive element. The conductive element is e.g. a rod- or cable-probe.
In physical terms, the TDR measurement method exploits the effect that, at the interface between which two different media meet, e.g. air and oil, or air and water, due to the abrupt change (discontinuity) of the dielectric constants of the two media, a portion of the guided electromagnetic, high-frequency pulses, or microwaves, is reflected, and guided via the conductive element back into a receiving apparatus. The greater the difference between the dielectric constants of the two media, the greater is the portion of the high-frequency pulses, or microwaves, reflected. Based on the travel time of the pulses or waves, the distance to the interface can be determined. If the empty distance of the container is known, the fill level of the fill substance can then be calculated.
TDR measuring devices with guided high-frequency measuring signals (pulses or waves) are distinguished by low signal damping, as compared with measuring devices, in which measuring signals are radiated freely. The reason for this is that the power flow occurs along the rod- or cable-probe, i.e. along a conductive element. Furthermore, TDR measuring devices have, even in the case of small measured distances, a high level of precision, because, when compared to measuring devices which freely radiate microwaves, they have a larger signal bandwidth, and therefore achieve better definition between useful, i.e. wanted, signals, and disturbance signals. A further advantage of TDR measuring devices lies in their high level of certainty and reliability of fill level measurement. This is because a measurement taken with guided measuring signals is relatively independent of the properties of the fill substance, the construction of the container (e.g. material, geometry), or other operating conditions (e.g. dust, accretion).
In addition, it has become known to determine the fill level of a fill substance in a container by means of a capacitive fill-level measuring device. Here also, a conductive element extends into the container. An alternating voltage signal from a signal source is applied to the conductive element. Via a measuring/evaluating circuit, the instantaneous, measured capacitance is established, and compared with a predetermined reference value for capacitance. On the basis of this data, the fill level of the fill substance is determined.
In capacitive methods for determining the fill level of a fill substance in a container, the capacitive probe and the container wall form the electrodes of a capacitor. In the case that the container wall is not conductive, a separate, second electrode must be provided inside or outside of the container. Depending on the fill level of the medium in the container, either the gaseous atmosphere of the container, or the fill substance, is located between the two electrodes. This is reflected in a change of the measured capacitance, based on the different dielectric constants of the two substances. Thus the measured capacitance shows a dependence on the particular fill level of the fill substance in the container. Capacitive probes can be used for detecting limit levels, as well as for continuously determining fill level. Additionally, conductive fill-level measuring devices also belong to the state of the art.
Devices of the type previously mentioned are used in a variety of ways in the pharmaceutical and foods industries, in the chemical industry, in electroplating and similar industries, and must fulfill a wide range of requirements in such applications.
The parts of these devices which touch the product are normally subject to considerable wear, for example through corrosion, abrasion, embrittlement, increase in hardness, surface cracking, or other deterioration. It is thus a great advantage, if the individual, wearing parts can be changed without needing to replace the entire measuring probe.
If these devices come into contact with food- or pharmaceutical products, then they must, for understandable reasons, meet the highest requirements for hygiene. The hygiene requirements for metrological devices are formulated by standards boards. In this connection, for example, there is the European standard EN 1672-2:1997, which has the status of a national German standard. This standard supplements the generally applicable, essential safety and health requirements of the EC Machine Directive 89/392/EEC of the Council of the European Communities by providing detailed requirements for machines used in the foods industry. On the same level with these standards is the state of the art, as published in the form of guidelines by groups of experts, such as the “EHEDG Guidelines” of the “European Hygienic Equipment Design Group”, or the “ASME-BPE” of the “American Society of Mechanical Engineers”. In these standards and guidelines, cleanability plays a central role, because such is essential for preventing health-endangering germs.
Potential collecting recesses for health-endangering germs tend to be found in areas in which two disengageable parts of a measuring apparatus are connected to each other. Especially critical are relatively narrow and tightly-dimensioned, interstitial spaces. A critical contact region, for example, is where the conductive element is attached on the measuring device. If there is a gap in the contact region between the conductive element and the measuring device, then the fill substance contained by the container can penetrate into the gap and deposit there. It is known that narrow gaps cannot be thoroughly cleaned, or can be only inadequately cleaned. A further problem with gaps is their tendency to corrode, so that, independenty of the hygienic requirements, gaps which come into contact with the fill substance must be prevented as much as possible.
In order to prevent this problem, it has become common to manufacture probes that are fully insulated with plastic (e.g. PFTE). These fully-insulated probes are very well-suited for hygienic applications, since, in this case, gaps which may come into contact with the fill substance are completely eliminated. However, these fully-insulated probes are expensive and susceptible to mechanical damage.
Furthermore, conical seals are also common, in which an insulating material is arranged in a bushing between the conductive element and housing. However, such conical seals are not permanently tight and free of gaps. The same is true for conical seals between the insulator and metallic adapter serving as the process connection. In addition, when replacing individual parts, such constructions can be disassembled only with great complexity since complicated spring mechanisms must be used to re-tighten the cones.